1. Field of the Invention
The present invention relates to a linearizer for use in conjunction with a power amplifier, and to a linearizer channel amplifier using this linearizer for predistorting an input signal in view of cancelling signal distortions subsequently produced by the power amplifier.
2. Brief Description of the Prior Art
Microwave power amplifiers (solid state or travelling wave amplifiers) used in ground station transmitters and communication satellites should ideally be highly efficient and provide linear amplification over a wide range of input power. Obviously, the performance of these microwave power amplifiers is limited by the non-linearities of the inner parts or constituents. A wide variety of correction methods for microwave power amplifier's non-linearities have been proposed and implemented (e.g., power back-off, negative feedback, feed forward, etc.). However, in most cases the efficiency of the linearized amplifier drops significantly relatively to the efficiency of the non-linearized amplifier specially when this amplifier input is a high crest factor modulated signal such as a CDMA (Code Division Multiple Access), NPR (Noise Power Ratio), or other types of signals.
Predistortion is one of the best cost-effective approaches to reduce the level of non-linear distortion generated by microwave power amplifiers. Predistortion consists of introducing in the input microwave signal amplitude and phase distortion opposite to the distortion produced by the microwave power amplifier to thereby cancel any distortion and obtain at the output of the power amplifier a signal that is an amplified replica of the input signal with almost no power efficiency decrease. Therefore, the function of a predistorter circuit is to generate both inverse amplitude and phase non-linearities. Since the characteristics of a microwave power amplifier significantly change with temperature, the predistortion circuit has to maintain the overall performance of linearity of the system and to follow the amplifier changes.